The effects of the low pressure system early this year on the sea level compared to other extreme events

The passage of several low pressure systems (2-5 January 2014) over a large part of western France caused a series of storm surges measured by the SHOM's RONIM tide gauges installed along the Atlantic coast. The maximum sea levels reached at each port during this storm were compared to the maximum water levels measured during previous major storms: Xynthia (02/28/2011), Quinten (02/9-10/2009), Johanna (03/10/2008) and Lothar-Martin (01/26-28/1999). This comparison shows that the sea levels measured at the beginning of January during this low pressure event were among the highest ever observed for the ports studied. The storm surge tides were within the average for this type of winter storm, ranging from 0.40 to 0.75 m. However, the strong tidal coefficients (108) that occurred at the same time explain the levels reached.

View of part of the port of Le Conquet, Finistère (N. Pouvreau, Photo credits SHOM - 3 January 2014)

View of part of the port of Le Conquet, Finistère (N. Pouvreau, Photo credits SHOM - 3 January 2014)

 

Data used: tide gauge observations

The data used are from the sea level heights measured by each tide gauge (in Le Conquet, Brest, Concarneau, Port Tudy, St. Nazaire, Les Sables d'Olonne, La Rochelle-La Pallice, Port-Bloc (Pointe de Grave) and Bayonne). The heights are expressed relative to chart datum. However, the more recent data were sampled every 10 minutes (averaged over 2 minutes) and the earlier data were sampled hourly, which could underestimate the maximum water level reached for the older events. Finally, note that these tide gauges are installed in stilling wells, which filter out some of the signal caused by waves and chop.

 

Comparison between the maximum water levels measured during the low pressure system early in 2014 and the maximum sea levels during benchmark storms.

The maximum water levels reached during the low pressure system in early January 2014 were compared to the maximum sea levels measured during the four benchmark storms (Xynthia, Quinten, Johanna, Lothar-Martin). Without equaling the storms Xynthia or Martin in December 1999, which were more powerful but also very local, the levels in Brittany were comparable to the benchmark storm Johanna of March 10, 2008. On the maps below, a 4-color code was used to indicate the following:

  • Dark blue: the maximum water level reached during the low pressure system at the beginning of the year was higher than the maximum sea level reached during the comparative storm (Xynthia, Quinten, Johanna, Lothar-Martin);
  • Yellow: the maximum water level reached during the low pressure system at the beginning of the year was equivalent (within uncertainties) to the maximum sea level reached during the comparative storm (Xynthia, Quinten, Johanna, Lothar-Martin);
  • Light blue (cyan): the maximum water level reached during the low pressure system at the beginning of the year was lower than the maximum sea level reached during the comparative storm;
  • Violet: no observations available during the comparative storm.

 

Comparison with storm Xynthia (28 February 2010)

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Xynthia - Click on image to enlarge

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Xynthia - Click on image to enlarge

 

Comparison with the storm Quinten (9-10 February 2009)

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Quinten - Click on image to enlarge

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Quinten - Click on image to enlarge

 

Comparison with storm Johanna (10 March 2008)

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Johanna - Click on image to enlarge

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storm Johanna - Click on image to enlarge

 

Comparison with storms Lothar and Martin (26-28 December 1999)

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storms Lothar and Martin - Click on image to enlarge

Maximum water levels reached during the low pressure system in early 2014 compared to the maximum sea levels measured during the storms Lothar and Martin - Click on image to enlarge

 

 

References

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